In some populations of birds, males may wonder why they can’t find a mate. It’s not that they’re unattractive or can’t sing the right song. It’s that females are in short supply.

This phenomenon is a common one in birds, particularly in threatened species and among populations that are small or fragmented. And scientists weren’t sure why this inequality crops up. Perhaps females are more likely to die, researchers speculated, or there are differences in when and where males and females move.

To figure out what might be happening to those missing females, Catriona Morrison and colleagues at the University of East Anglia in Norwich, England, turned to a small migratory bird, the willow warbler. These warblers migrate from across the northern parts of Europe and Asia to sub-Saharan Africa and back, and they can be found throughout the British Isles. The researchers gathered data from a British program for which volunteers count birds throughout the country, including counts of some 8,000 warblers made at 34 sites from 1994 to 2012.

In 1994, the warbler populations had about equal numbers of males and females. But by 2012, that had changed at some sites, especially those in the south and east of England. At some of those, males outnumbered females by as much as 2 to 1.

Differences in survival couldn’t explain the inequality. At one of the larger sites, males did have better survival rates than females, but everywhere else, the two sexes survived at about the same rate. Instead, the skewed ratios probably arise from where the birds go — or don’t go — when they reach adulthood, Morrison and colleagues report July 8 in the Journal of Animal Ecology.

When willow warblers get old enough, the females not only leave the nest but their entire home population. (This is something that happens in many bird species.) Males, though, tend to stay behind. The male-dominated populations may arise because females are choosing to join populations that are bigger — and have more males for them to choose from or more males that they find attractive — or ones that are situated in a better habitat. The smaller, fragmented populations just can’t attract as many females.

Having too few females has consequences for the bird population. If males can’t find mates and breed, they can’t produce offspring. This might push a declining population to decline even faster. For that reason, it might be better to focus conservation efforts on preserving habitat for large populations, the researchers suggest.

The findings also have implications for how scientists monitor bird populations. Since counting individual birds can be time consuming and expensive, monitoring programs often use a substitute — male song. But among willow warblers, males sing less after they have mated. So if there is a male-dominated population with a lot of guys that never get to mate, that abundance of song might be misleading.